1. Field of the Invention
The present invention relates to a process and apparatus for monitoring or adjusting, or both, the concentration of total reducers in a water medium.
2. Brief Description of the Prior Art
Oil refineries and other industrial complexes often have effluent waste water streams which contain high concentrations of reduced sulfur compounds including sulfides (S.sup.-2), thiosulfates (S.sub.2 O.sub.3.sup.-2) and sulfites (SO.sub.3.sup.-2). Such waste water streams must be chemically treated or placed in bioponds before being discharged into public waterways or going to public owned treatment works in order to comply with discharge permits. One common method of treating these sulfur-bearing waste water streams is by adding to the stream an oxidizing chemical such as hydrogen peroxide, chlorine dioxide, potassium permanganate, chlorine gas, sodium hypochlorite or ozone.
In the past, the required amount of oxidizing chemical to be added to the waste water stream was determined by first analyzing a batch sample of the stream. This batch analysis was then related to the flow rate of the stream and the final treated pollutant level desired. The chemical oxidant feed rate was manually adjusted following every such determination.
The standard laboratory analysis for measuring the aggregate concentration of reduced sulfur compounds was a batch iodometric analysis. This analysis is sometimes referred to as a Total Reducers analysis. Such an analysis attempts to determine the aggregate concentration of all of the reduced sulfur compounds and other chemicals which consume iodine. One particular known batch method involves first adding four chemical reagents (i.e. a pH 5.0-5.5 phosphate buffer, a methyl red indicator solution, hydrochloric acid and a starch indicator solution) to a waste water sample. Then, an iodine solution is titrated into the mixture of the waste water sample and chemical reagents until a color change occurs. The reduced sulfur components in the water sample quantitatively react with the iodine titrant under certain acidic conditions (i.e. pH 5.0-5.5). When all of these reduced species are consumed by this reaction, the iodine starts to react with the starch indicator to form a blue complex signaling the end point of the titration. Upon reaching the endpoint, the total amount of titrated iodine is measured and the concentration of total reducers as milligrams of thiosulfate as sulfur or sulfite as sulfur per liter of water is calculated therefrom.
Typical batch analysis procedures are illustrated in APHA Standard Methods 13th Edition, 1971, pages 337 and 338 and the Modified Los Angeles County Sanitation District Method September 1985 and Olin Water Services Analytical Method for Total Reducers Titration (dated November 1985). All three of these procedures are incorporated herein by reference in their entireties.
Until the present invention, no one had attempted to automate any standard batch iodometric analysis so that the total reducers level in a waste water stream could be continuously monitored. Furthermore, no Total Reducers analytical system has been used to automatically control process adjustment means (e.g. oxidant chemical feed pumps).
Technicon Industrial Systems of Terrytown, New York, has manufactured an on-line hydrogen sulfide monitor (Model 650) for the intermittent (once per hour) detection of hydrogen sulfide (H.sub.2 S) content in an aqueous stream using a colorimetric methodology of detection. However, this Technicon monitor does not measure thiosulfate and other total reducer constituents in an aqueous stream.
Dionex Corporation of Sunnyvale, Calif., manufactures an on-line monitor (Model 8000) which is capable of detecting the concentration of the individual anion and cation components in an aqueous stream with ion exchange methodology. By this method, a small sample of a water stream is passed through an ion exchange column and the various ionic constituents are loosely attached by the ionic charge to the resin. During regeneration of the resin column, the various ionic contituents are eluted in a specific order and may be qualitatively identified and quantitatively measured by ion chromatography.
Both the Technicon and Dionex systems are very sensitive to the presence of suspended solids and oils in the aqueous sample being tested. Accordingly, both use a submicron filter to pretreat the aqueous samples being tested. This pretreatment of the sample may easily introduce errors into the analysis procedure by removing a portion of the chemical constituents being analyzed. Accordingly, these Technicon and Dionex monitors are best used for analyzing fresh water and laboratory water. Neither is well suited for waste water analysis. Finally, neither monitor is designed to provide an output signal based on an monitored combination of a variable flow rate and a total reducers concentration to a process control means (e.g. chemical feed pump) which controls the level of the oxidant reactant feed into a waste water stream.
Accordingly, there is a need in the water treatment industry for an automated Total Reducers analytical system which can provide a very frequent analysis of total reducers concentration in a water stream without prefiltering or other preconditioning means. Furthermore, there is a need in this industry for an automated Total Reducers analytical system, when coupled with a flow monitoring means, that can automatically control a process adjustment means (e.g. oxidant chemical feed pump), especially when a variable flow rate of water stream is involved. The present invention is a solution to both of these needs.